Hi again!
I posted my Nishiki in this forum a while back where you can see all the info I know about the bike. I got a response from pitbull007 here and with catalog scans here. Thanks for those!

However, though the catalog scan for the International looks very similar to mine, it's not quite the same. Notably the decals and coloring on the seat tube.
So I'm still not sure if it's a 1976 or a 1986. Looks very much like the 1986, but the date codes on the parts seem to indicate 1976. The 1986 was all Hi-Ten tubing (according to catalog) but I'm not convinced my bike is Hi-Ten. The frame is lighter than I'd expect with Hi-Ten and the main tubes give a nice 'ring' when flicked. The stays and fork, however, give a dull 'thunk.'

Also still wondering about this "WILL" stamped on the stem.

Anyway, I've recently switched up the bike a bit to make it more of a relaxed ride. I've added photos to the Flickr set here. The last 4 photos (taken indoors) are of my current set-up. I'm now looking at putting wider rims and tires on it, with a wider 'comfort' saddle.